This invention relates to a method for producing glow polymerisate layers upon a substrate from monomeric hydrocarbons and/or fluorocarbons by means of a high frequency low-pressure glow discharge.
Glow polymerization in which polymers are produced from gaseous organic monomers on a substrate by low-pressure plasma excitation, permits the preparation of thin, uniform and porefree layers. This method also permits use of monomers which can not otherwise be polymerized. Particularly advantageous means of generating low-pressure plasmas for glow polymerization are high-frequency discharges, specifically those in the radio-frequency range (see: A. T- Bell in S. Veprek, M. Venugopalan, "Plasma Chemistry III", Springer Verlag Berlin Heidelberg, 1980, pages 43 ff.), i.e. in the range between 0.1 and 100 MHz (RF), and those in the microwave range (see: "J. Macromol. Sci.-Chem." A 14 (3), pages 321 to 337, 1980), i.e. in the range between 0.1 and 1000 GHz (MW). In these ranges the energy can be coupled capacitively or inductively into the plasma (i.e. into the reactor in which the glow polymerization is performed) without difficulty by use of external lines, or by hollow conductors such as resonator cavities, the so-called "slow wave structures", etc., whereby a reactive interaction between the electrically conducting parts and the plasma is avoided.
An interesting aspect in connection with glow polymerization is the manufacture of layers which are formed from hydrocarbon (CH) and/or fluorocarbon (CF) monomers. It should be possible to produce from these organic monomers, which comprise a broad spectrum of polymerizable compounds, glow polymerisates with a wide range of properties that can be selected with great specificity. For instance, the production of thin layers on a CH and/or CF basis for low-loss dielectrics; coatings which have low-energy surfaces and thereby, liquid-repellant coatings; sensitive resist layers for dry structuring processes in x-ray and electron beam lithography; thin film diaphragms for electromedical applications and electrets for various applications is a possible use of this technique. It should also be possible to produce these compositions with quantitatively and qualitatively high yields.
However, if high-frequency RF and MW glow discharges are used in the polymerization of CH, CF and CH--CF monomers, the resulting polymers have an H and/or F content which is lower than that of the monomers. The reason for this is that H and F atoms as well as species rich in H and F atoms are split off in the plasmas from the monomer molecules by collision with charged particles, particularly the electrons, and are removed with the gas stream of the monomer component and are thereby lost. As a result, the layers which are formed have a less than stoichometric C/H or C/F composition. Primary unsaturated structures and radicals are thereby formed in the layers which can react oxidatively in the presence of air or oxygen and produce undesirable, undefined polar structures. These structures cause, for instance, a degradation of dielectric properties, an increase in the water absorption and a chemically changed behavior in the action of solvents (lyophility).